Find the range of values of xx for which 2x(x−3)≥3−5x2x(x-3) \ge 3-5x. Find the greatest value of kk such that the line 2y+k=x2y+k=x does not intersect the curve y=2x+6xy=2x+\frac{6}{x}, where kk is an integer.

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Worked solution

6(b)

1. Express $y$ in terms of $x$ from the linear equation and substitute into the curve equation

From 2y+k=x2y + k = x, we have y=x−k2y = \frac{x - k}{2}.
Substitute this into y=2x+6xy = 2x + \frac{6}{x}:
x−k2=2x+6x\frac{x - k}{2} = 2x + \frac{6}{x}

2. Rearrange the equation into a standard quadratic form $ax^2 + bx + c = 0$

Multiply throughout by 2x2x:
x(x−k)=4x2+12x(x - k) = 4x^2 + 12
x2−kx=4x2+12x^2 - kx = 4x^2 + 12
3x2+kx+12=03x^2 + kx + 12 = 0

3. Apply the discriminant condition for no intersection

For the line to not intersect the curve, the quadratic equation must have no real roots, so b2−4ac<0b^2 - 4ac < 0.
k2−4(3)(12)<0k^2 - 4(3)(12) < 0
k2−144<0k^2 - 144 < 0

4. Solve the inequality for $k$ and find the greatest integer value

(k−12)(k+12)<0(k - 12)(k + 12) < 0
−12<k<12-12 < k < 12
Since kk is an integer, the greatest value of kk is 1111.

Answer

k=11k = 11

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